1. Field of the Invention
The present invention relates to radar antenna stabilization error correction and more particularly to the correction of attitude sensor errors as well as antenna beam elevation errors.
2. Description of the Prior Art
Airborne radar systems used to detect and annunciate flight hazards require means to accurately point the antenna beam with respect to the local earth reference. It is common practice in both air transport and general aviation radar systems to obtain aircraft attitude data, i.e. pitch and roll information, from an attitude reference sensor external to the radar system. Such an attitude sensor may be an inertial reference system, a vertical gyro, or a bank and pitch instrument containing a vertical gyro. Attitude sensors are known to exhibit both fixed and time varying errors which degrade antenna beam pointing accuracy; sensor errors of 2.degree. to 3.degree. are not uncommon with vertical gyro based instruments.
Antenna beam pointing is also effected by errors internal to the radar system, e.g. positioner calibration, mechanical versus electrical boresight alignment of the antenna, and droop in the positioner mechanism. In general, for normal aircraft maneuvers, these errors are independent of aircraft attitude. Internal errors are also independent of both small antenna elevation angles and azimuth angles. Elevation angle is the vertical angle orthogonal to, and measured from, the plane defined by the aircraft longitudinal and lateral axis to the boresight of the antenna beam. Azimuth angle is the horizontal angle measured in the plane defined by the aircraft longitudinal and lateral axis and measured from the aircraft longitudinal axis to the position of the antenna beam projected onto the measurement plane. The above mentioned internal errors are all manifested as beam elevation errors, and as such, will be referred to hereafter as elevation errors.
Current practice for radars used in the air transport and general aviation community is to provide the operator with a means for manual adjustment of radar antenna beam tilt. Such adjustment allows the operator to manually compensate for beam elevation errors and some attitude sensor errors. Some radar systems also provide separate means for manual adjustment for pitch trim and/or roll trim.
It is an object of the present invention to provide a system that automatically estimates and corrects attitude sensor errors in pitch and roll and antenna beam elevation errors.